$1, \mathrm{~m}, \mathrm{n}$, are the direction cosines of a straight line them (A) $l^{2}+m^{2}-n^{2}=1$ (B) $l^{2}-m^{2}+n^{2}=1$ (C) $l^{2}-m^{2}-n^{2}-1$ (D) $l^{2}+m^{2}+n^{2}=1$